Maya Ellison
Whether or not you should install plastic over insulation is a complex question that depends on a variety of factors, including the type of insulation, the climate, and the specific building. Some people recommend installing plastic sheeting (often referred to as visqueen or poly) as a vapour barrier to prevent moisture damage, especially in cold climates. However, others caution that plastic vapour barriers can actually trap moisture and cause condensation, leading to rot and mould problems. The key consideration is understanding the direction of airflow and moisture movement in your building and ensuring that any vapour barrier does not prevent drying to the drier space, whether that is indoors or outdoors depending on the climate.
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What You'll Learn
Pros and cons of plastic over insulation
Plastic over insulation acts as a vapour barrier, which can be beneficial or detrimental depending on the context.
Pros of Plastic Over Insulation:
- In cold climates, a vapour barrier can prevent water vapour from entering wall cavities and causing condensation, which can lead to mould and rot.
- In cold weather, a vapour barrier on the interior side of a wall can keep water vapour from the humid indoor air away from the cold surfaces inside the wall, preventing moisture problems.
- Plastic over insulation can cut down on drafts into the house.
Cons of Plastic Over Insulation:
- In warm, humid weather, a vapour barrier can prevent drying to the indoor space, trapping moisture and leading to mould and rot problems.
- If the wall allows outdoor air to infiltrate and the plastic is below the dew point, condensation can form and cause water damage to the wall.
- Plastic vapour barriers can interfere with the wall's ability to breathe, which can be detrimental in some situations as it may be better to let the walls dry out rather than trapping moisture.
- Plastic vapour barriers may not be necessary with certain types of insulation, such as kraft-faced insulation, which acts as its own vapour barrier.
- Plastic vapour barriers can be difficult to install correctly, requiring all seams to be taped shut, and may not be worth the extra time and material cost.
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The importance of climate
The decision to install plastic over insulation is influenced by several factors, including the local climate, which plays a crucial role in determining the required insulation R-value (thermal resistance) and the choice of materials.
In freezing climates, installing a vapour barrier on the warm side of a wall is standard practice, as it prevents moisture from infiltrating the insulation and causing damage. However, in areas with significant temperature fluctuations, such as deserts, the decision to install plastic may differ.
In mixed humid climates, like most of the eastern US, it is recommended to have a vapour retarder in the middle of the thermal sandwich. This can be achieved using continuous foam boards, fibrous insulation, and building felt on wood sheathing.
In coastal areas with high precipitation and frequent freeze-thaw cycles, the philosophy is often to let the walls breathe so they can dry out effectively. In such cases, people tend to avoid vapour barriers altogether.
The type of plastic insulation used also depends on the climate. For instance, in below-grade applications like basements and foundations, extruded polystyrene (XPS) is used due to its moisture resistance and high compressive strength. On the other hand, in areas with extreme temperature variations, polyurethane (PU) foam is popular due to its excellent thermal resistance.
In conclusion, the importance of climate cannot be overstated when considering the installation of plastic over insulation. Climate influences the choice of materials, the required R-values, and the overall effectiveness of the insulation system. By taking into account the specific climatic conditions, homeowners can make informed decisions about the use of plastic vapour barriers and select the most suitable insulation materials for their region.
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Vapor barriers and condensation
Vapor barriers are materials used to resist the diffusion of moisture through the walls, ceilings, or floors of a building. They are crucial in maintaining a comfortable indoor climate and saving energy. By preventing moisture damage, vapor barriers protect your insulation and preserve the integrity of your home.
Vapor barriers are especially important in extremely cold climates, such as Alaska, Canada, or Montana. In these regions, warm, moist air can move through walls and condense on the first cold structure it encounters. A vapor barrier installed on the inside of a building can help prevent this condensation.
However, vapor barriers can also cause condensation issues if not properly installed or used in conjunction with other materials. For example, if there are gaps or leaks in the vapor barrier, interior air carrying water vapor can leak through and condense when it reaches cooler temperatures. This can lead to moisture buildup and potential mold or structural issues.
To avoid condensation problems, it is essential to install vapor barriers correctly. This includes ensuring the vapor barrier faces the warm side of the insulation, sealing all seams, and paying close attention to edges and corners, which are often weak spots for moisture intrusion. Additionally, it is crucial to consider the specific climate and construction of your home when deciding whether to install a vapor barrier and what type to use.
In some cases, alternative methods such as using impermeable insulation or a "wrong-side" vapor barrier may be more suitable. A "wrong-side" vapor barrier involves sealing the assembly on the cold side and keeping it vapor open on the warm side, preventing humid air from contacting cold surfaces where condensation can occur. Ultimately, the decision of whether to install plastic over insulation and how to manage condensation depends on various factors, including climate, construction, and the specific needs of the building.
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Installation techniques
Before installing plastic over insulation, it is crucial to determine the dew point of the walls and understand the specific requirements for your region. Consult with professionals who can guide you through the process and ensure that your installation is safe and effective.
When installing plastic as a vapour barrier, it is essential to cut around windows, doors, and electrical outlets. Seal the edges with a caulk or sealant designed for use with foam to prevent degradation of the material. Install an unbroken sheet of polyethylene vapour barrier, commonly called poly or plastic sheeting, which can range in thickness from 4 mil to 20 mil. Cover any seams, holes, or tears with insulation tape designed for vapour barrier use. Apply a bead of acoustical caulk sealant along the bottom and top of the wall, where the plastic meets the floor or ceiling. Ensure good contact by pressing the poly firmly into the sealant.
For fiberglass batt insulation, some professionals recommend using unfaced batts and covering them in plastic with taped seams. This method helps prevent moisture from entering the wall cavities. However, other contractors prefer using faced batts with kraft paper or foil facing, eliminating the need for a separate vapour barrier.
In high-moisture areas like basements or crawl spaces, adding a separate plastic vapour barrier can provide extra protection. Check local building codes, as some regions have specific moisture control requirements. Remember that the vapour barrier should face the warm side, typically the interior of the wall, but this may vary depending on local building codes.
Additionally, consider the climate when installing plastic over insulation. In very hot and humid climates with air conditioning, a vapour barrier could potentially create issues. In such cases, construction products designed to allow some moisture vapour diffusion may be more suitable.
Proper installation is critical to ensuring the effectiveness of the insulation and preventing issues such as mould, rot, and reduced energy efficiency. Always follow safety guidelines and consult professionals when in doubt.
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Alternative methods
Plastic is sometimes used as a vapour barrier to prevent condensation in the wall cavity. However, this is not always effective, and there are alternative methods to covering insulation.
Firstly, it is important to note that the chosen material should be permeable to prevent moisture from being trapped, which can lead to mould and mildew growth.
One alternative is to use foam board insulation, which has high thermal and energy efficiency, as well as moisture resistance. It is lightweight, durable, and easy to install. Another option is to use foam-insulation products that come with protective finishes, such as WallGuard, which is made of concrete and attaches with clips. Stone and brick veneers are another option, which involves attaching galvanized lath through the foam to the foundation with concrete screws and washers, then applying a scratch coat of mortar and the veneer.
For a quick and inexpensive solution, one can use large sheets of gypsum board (also known as sheetrock or gyprock), which can be painted to improve aesthetics. Bamboo yoga mats are another cheap and easy-to-install option.
For those living in areas with termites, it is important to look for foam that is treated to resist insects and to install termite flashing.
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Frequently asked questions
A vapor barrier is a sheet of plastic or kraft paper that keeps water vapour out of a wall cavity, so the insulation stays dry.
It depends on the climate. In cold weather, a sheet of plastic on the interior side of a wall probably won't cause any problems. However, in warmer climates, it is best to avoid plastic barriers unless you live in an extremely cold region.
If the wall cavity gets wet, the plastic can prevent it from drying out, leading to mould and rot.
